Goodgulf:
There's a comma there... It just doesn't work for me. But then again, commas are the punctuation with the vaguest rules. How does it look when it's moved and other punctuation takes its place?
I am never sure of the punctuation, the rules are too complex. How I muddle through is I think of it as how I would say it and put commas, or sometimes semicolons, in where I would naturally pause but not come to a full stop.
Probably this would be better: She appeared unsure of herself; coy, as girls who aren't sure if they have become women yet frequently are.
Speaking it I wouldn't pause between "yet" and "frequently".
I often revise the punctuation after I have proofread what I have written.